1. Field of the Invention
The present invention relates to an improved process used in the finishing of die rolls used in the manufacture of interlayers for laminated safety glass. More particularly, the invention relates to a method for reducing the peaks in the surface of the roll without substantial reduction of the valleys.
2. Description of the Prior Art
Laminated safety glass is well known in the art for use as glazing units for vehicles, especially automobile windshields, and for architectural uses. Laminated safety glass comprises at least one sheet of glass laminated to at least one sheet of a synthetic resin material which is usually referred to as the interlayer. The laminate may be the sandwich type construction wherein the interlayer is interposed between two pieces of glass. Alternately, the laminate may comprise a single sheet of glass laminated to a synthetic resin sheet (also referred to as an interlayer for the purposes of the present invention). In the latter type of laminated safety glass the surface of the synthetic resin sheet, which is not laminated to the glass, may optionally be coated with or laminated to another synthetic polymeric material.
Interlayers are conventionally prepared with rough surfaces, which may be the same or different for both sides of the sheet.
In die roll extrusion methods for preparing interlayers, molten polymeric material is cast onto a die roll. The die roll surface is specially prepared in order to achieve a desired degree of roughness. Sheet cast onto this rough surface then acquires a rough surface from the imprint of the die roll. The peaks and valleys on the surface of the sheet correspond to the valleys and peaks, respectively, on the surface of the die roll. The rough surfaces of the die roll and the sheet are defined herein in terms of peaks and valleys occurring around a roughness means centerline (hereinafter defined). The rough surfaces on the sheet are required in order to prevent blocking, i.e., the tendency of the interlayer to stick to itself. Rough surfaces are also required to prevent premature adhesion of the interlayer to the other component of the laminate, usually glass, and to facilitate de-airing of the laminate assembly during the lamination procedure.
Although the surface roughness helps to prevent blocking and facilitates de-airing, it causes problems in the printing of the surface of the sheet. The rougher the sheet the harder it is to obtain an acceptable printed gradient on the sheet. The deeper valleys in the sheet are especially troublesome in that they give rise to significant printing defects. Consequently, seasoning of the sheet, i.e., heating of the sheet at elevated temperatures for prolonged periods of time is required in order to obtain a uniform gradient.
Interlayer materials cast onto the die rolls of the prior art exhibit adequate surface roughness in regard to the anti-blocking and de-airing characteristics of the sheet. However, these sheets have deep valleys which give rise to printing defects which require seasoning in order to reduce or eliminate the defects. The deep valleys in the sheet are caused by the high peaks on the surface of the die roll.
A need exists in the art for an improved method for preparing die rolls and for the die rolls made by this method in order to produce interlayer materials with fewer deep valleys.